1. Field of the Invention
The present invention relates to a method of producing an L-amino acid using a bacterium, and more particularly, to a method of producing an L-amino acid such as L-threonine and L-phenylalanine. L-threonine is useful as an additive in animal feeds, health food, amino acid infusions, and the like. L-phenylalanine is useful as a precursor for synthesizing alpha-L-aspartyl-L-phenylalanine.
2. Brief Description of the Related Art
L-amino acids are industrially produced by fermentation using various microorganisms. For example, L-glutamic acid is typically produced by fermentation utilizing the so-called coryneform bacteria, which belong to the genus Brevibacterium, Corynebacterium or Microbacterium, or mutant strains thereof (Kunihiko Akashi et al., “Amino acid fermentation”, pp. 195-215, 1986, Japan Scientific Societies Press). Methods for producing L-glutamic acid by fermentation using other bacterial strains include methods using a microorganism belonging to the genus Bacillus, Streptomyces, Penicillium or the like (U.S. Pat. No. 3,220,929), methods using a microorganism belonging to the genus Pseudomonas, Arthrobacter, Serratia, Candida or the like (U.S. Pat. No. 3,563,857), methods using a microorganism belonging to the genus Bacillus, Pseudomonas, Serratia, Aerobacter aerogenes (currently referred to as Enterobacter aerogenes) or the like (Japanese Patent Publication (KOKOKU) No. 32-9393), methods using a mutant strain of Escherichia coli (Japanese Patent Laid-open (KOKAI) No. 5-244970), and so forth. In addition, methods for producing L-glutamic acid using a microorganism belonging to the genus Klebsiella, Erwinia, Pantoea or Enterobacter have also been disclosed (Japanese Patent Laid-open Nos. 2000-106869, 2000-189169, and 2000-189175).
Methods for producing a target substance such as L-amino acid using such bacteria include methods using wild-type bacteria, methods using nutrient-auxotrophic mutants derived from wild-type bacteria, methods using metabolic regulation mutants derived from wild-type bacteria, and methods using mutants that have properties of a nutrient auxotrophic mutant and a metabolic regulation mutant.
Recently, DNA recombination techniques have been used for fermentative production of target substances. For example, L-amino acid productivity was improved by enhancing expression of genes encoding L-amino acid biosynthetic enzymes (U.S. Pat. No. 5,168,056 and U.S. Pat. No. 5,776,736), or by enhancing carbon influx into L-amino acid biosynthetic pathway (U.S. Pat. No. 5,906,925).
Huang Y et al. (FEMS Microbiol Lett 275 (2007) 8-15) reported that the yggG gene encodes a membrane-localized heat-shock protein with a molecular weight of about 25 kDa. This protein is predicted to be a metallopeptidase based on its sequence information, and that the yggG gene product interacts with a Era protein, which is GTPase. Huang Y et al. also reported that expression of the yggG gene is increased in an era-mutant strain and era-overexpressing strain (Curr Microbiol. 2008 January; 56(1):14-20. Epub 2007 October 2). However, the physiological function of the yggG gene is unknown and there have been no reports on the relationship between enhancing yggG gene expression and L-amino acid production.